Nobody is a special value used to indicate that the agent you were looking for does not exist. When a turtle dies, it becomes equal to nobody. Nobody is often used to check that there is an agent in an agentset, to avoid an error. For example, you might include

if one-of turtles with [color = red] != nobody [ 
   ask turtles with [color = red ] forward 1 ]

so in the case that there weren’t any red turtles, you would prevent an error.

It is important to note that an empty agentset is not equal to nobody; nobody is only used when looking for a single agent, usually in the context of turtle, one-of, or max-one-of.